PROJECT SUMMARY Although an understanding of the biological mechanisms causing scleroderma (SSc) remains incomplete, emerging data underscore important connections linking cancer, autoimmunity and SSc. It is an important priority to identify SSc subgroups at high risk of cancer at SSc onset, as this may inform therapeutic approaches and provide insights into SSc pathogenesis. Our recent studies show that autoantibodies are useful to define biologically relevant subgroups. Among SSc patients with cancer, those with anti-RNA polymerase III (POLR3) antibodies are more likely to have cancer close to SSc onset (3, 4). However, >25% of patients with a short cancer-SSc interval currently have undefined autoantibodies. We have created an innovative autoantibody discovery pipeline, and demonstrate that multiple, additional autoantibodies (known and novel) are associated with coincident cancer at SSc onset. We also describe a novel antibody specificity that is enriched in anti-POLR3-positive patients who do not get cancer, suggesting that some immune responses might be protective against cancer. Lastly, we demonstrate that autoantibody subsets can aid in defining the magnitude and timing of cancer risk in SSc, and in predicting the types of tumors observed. The proposed studies will use a large population of SSc patients from two well-characterized SSc cohorts to define and validate autoantibodies associated with increased or decreased cancer risk, and to assess their utility, together with distinct phenotypic features, in quantifying cancer risk at SSc onset. We will accomplish this through the following specific aims: Aim 1 will use an innovative antibody discovery pipeline and a rich cohort of SSc patients with cancer to identify known and novel autoantibody markers of a short cancer-SSc interval in patients lacking the 3 major SSc antibody specificities. The studies will also use the defined autoantibodies to search for evidence of somatic mutations of relevant autoantigens in matched cancers. Since ~80% of patients with POLR3 antibodies do not have cancer, and our preliminary data shows novel antibodies in this group, we will also identify autoantibodies associated with protection against cancer. These may be important markers of low cancer risk. Aim 2 will define time-dependent overall and site-specific cancer risk in patients with SSc relative to the general population, based on autoantibody and phenotypic subsets, to develop an evidence-based approach to cancer screening in patients with new onset SSc. Findings will be validated in a second large SSc cohort. This work will allow the development of a clinically relevant approach to cancer detection at SSc onset and establish a framework of more homogeneous serologic subgroups in whom the mechanistic relationships between cancer and autoimmunity can be interrogated. This approach is also relevant to other autoimmune rheumatic diseases (e.g. inflammatory myopathies) in which cancer and rheumatic disease onset are temporally linked.